The present invention is about alkali or alkaline-earth metal dispensers stable to environmental gases, in particular air, especially adapted for use in the fabrication of miniaturized devices.
A number of industrial applications require the presence of alkali or alkaline-earth metals in different physical forms, e.g., in the form of thin solid films deposited onto a surface of a device or in the form of vapors. Among these, one can remember photocathodes, in which the active element is a surface made of an alkali metal (or of an intermetallic compound containing an alkali metal); CRTs, in which a deposit of an alkaline-earth metal (typically barium) on the inner surface of the tube acts as a trap for gases, keeping the required degree of vacuum inside the same tube; atomic clocks, in which an electromagnetic radiation is passed through vapors of an alkali metal (rubidium or, more commonly, cesium); atomic interferometers, described in International Patent Application No. WO 2006/084113, and atomic gyroscopes, described in European patent application No. EP 1865283; and refrigeration units based on the tunnel effect, in which cooling is due to transport of electrons between a cathode and an anode, and a deposit of an alkali metal on at least the electron-emitting surface of the cathode helps reduce the work function of the cathode and consequently the energy required for operating the system. Detailed information about this mechanism, called “thermotunneling”, can be found in the article “Refrigeration by combined tunneling and thermionic emission in vacuum: use of nanometer scale design” of Y. Hishinuma et al., published in Applied Physics Letters, vol. 78, no. 17, pages 2572-2574 (2001), while an example of use in an actual device is given in U.S. Pat. No. 6,876,123 B2.
Alkali or alkaline-earth metals are not easy to handle or ship due to their high reactivity towards atmospheric gases and moisture. Dispensers of these metals, used for a long time, contain them in the form of stable compounds. Dispensers of alkali metals, in which these metals are present in form of their salts (e.g., chromates, vanadates, titanates and similar) are described, for instance, in U.S. Pat. Nos. 3,579,459 and 6,753,648 B2, and in European patent application No. EP 1598844 A1; dispensers of barium, containing the stable compound BaAl4, are described in a number of patents including, to cite but a few, U.S. Pat. Nos. 2,824,640 and 4,642,516; dispensers of calcium, containing the compound CaAl2, are described e.g., in U.S. Pat. No. 6,583,559 B1.
All of the dispensers disclosed in the above cited documents are however bulky, and not suitable for use in the production of, or for insertion in, miniaturized devices, such as for instance the thermotunneling refrigerating units described in the Hishinuma article cited above, or in miniaturized atomic clocks, such as those described in the paper “Microfabricated alkali atom vapor cells” of Li-Anne Liew et al. (Applied Physics Letters, vol. 84, no. 14, pages 2694-2696 (2004)).
For their proper working, it is necessary for the previously cited industrial applications that the inner cavity of the devices be kept under vacuum or anyway free from reactive gases. In the case of a thermotunneling refrigerating unit, the presence of gases between the cathode and the anode could hinder the traveling of electrons, and could cause the back-transfer of heat by convection. These units generally require a vacuum better than 10−1 hectoPascal (hPa) and preferably in the range of 10−4 hPa. In the case of atomic clocks, gases present in the cavity could react with the vapors of the alkali metal, thus causing the diminishing of the amount of free metal vapor and worsening of the working of the clock. Despite the fact that the manufacturing processes of these (and other) devices commonly comprise steps of evacuation of the cavities, phenomena like permeation from outside, leaks, and outgassing from the surfaces of said cavities reintroduce undesired gases in the same during the device life. In order to tackle this problem, it is known to add getter materials inside cavities, that is, materials capable of chemically reacting and thus strongly fixing gaseous species. Getter materials are generally metals like titanium, zirconium, vanadium, hafnium or niobium, or alloys of these (and mainly of titanium and/or zirconium) with one or more metals chosen among transition elements, Rare Earths and aluminum.